Unexpected email and Alexa bot

I got a surprise today when I found that I have an email in my inbox from webmaster@animorphsfanforum.com. The email had subject “Subject” and has content “Content”. My first instinct was that someone has hacked into my VPS. After a few minutes of mind-racking, I remembered that I had created a PHP file on the VPS named ‘mail.php’ to test whether emails were being sent. But, no one knew of it’s existence except me. Someone must’ve stumbled upon it my chance.